Surveying of boreholes, such as those used in geologic surveying, mining and oil well drilling requires an accurate determination of the azimuth and elevation coordinates of the boreholes so that an accurate plot of the direction and depth of the borehole can be made. Surveying of a borehole is often accomplished by an instrument or a probe which moves through the borehole and measures inclination and azimuth angles at successive points. Inclination, the angle by which the borehole deviates from the vertical, may be measured with a pendulum or an accelerometer. Azimuth, the angle of the borehole with respect to reference direction, such as north, is typically measured with a magnetic or gyroscopic compass. These angles, together with the distance along the borehole, are used to determine the coordinates of points along the borehole with respect to a reference on the ground.
Various approaches have been used in surveying boreholes in the past including the use of magnetometers, gyroscopes and accelerometers. For example, a pendulum for measuring inclination may take the form of a linear servoed accelerometer which responds to gravity. Servoed accelerometers are available which are small, rugged and accurate. The accurate measurement of azimuth can be quite difficult, however. For instance, magnetic compasses or other devices for measuring the earth's magnetic field are subject to errors caused by magnetic anomalies in the ground. Gyroscopic compasses also have several drawbacks including large size, bearing wear, sensitivity to shock, drift and precession errors and the requirement for a long settling period for stabilization when a measurement is made. As a result, borehole surveying instruments utilizing gyroscopes tend to be expensive and complicated as well as requiring probes with a large diameter.
An example of another approach is provided in the copending patent application to Liu entitled "Borehole Survey Apparatus and Method", Ser. No. 200,096, filed on Oct. 23, 1980 in which a probe having two sections connected by a torsionally rigid member has an accelerometer package in each probe which are utilized to derive the relative tilt and azimuth angle of the borehole as the probe descends in the borehole. This approach has a significant advantage over prior art methods of borehole surveying in speed and accuracy and the further advantage of not having to utilize a compass for azimuth measurement. In addition, because it utilizes accelerometers, the probe may have a relatively small diameter housing and is substantially more rugged. However, this particular method has as one of its disadvantages the inability to determine azimuth when the direction of the borehole is very close to horizontal.